This invention relates to solar or photovoltaic cells, and more particularly to that variety of solar cell wherein a plurality of elongate and closely spaced cell units is formed as an array of such units, the formation being from the same single wafer of semiconductor substrate material.
The embodiments of the invention here described involve a solar cell which has an n-type silicon substrate. It is to be understood, however, that the solar cell of the invention may also be implemented with a substrate of p-type polarity, in which case the polarity of other regions of the cell would be reversed, with n replaced by p, n+ replaced by p+, and so on. It will also be understood that other types of semiconductor material may be employed and that a heterojunction, as well as the homojunction structure here described, may be employed.
The concept of a multiple-unit array of solar-cell units formed from the same single wafer of substrate material is disclosed in our U.S. Pat. No. 4,110,122, wherein the body material of each unit advantageously has the same positional relationship as existed in the original wafer; and methods of making such arrays are the subject of our U.S. Pat. No. 4,131,984. And in U.S. Pat. Nos. 4,128,732 and 4,129,458, there are disclosed various array structures of the single-wafer variety which seek to enhance the electrical output and efficiency of each unit (a) by presenting maximum exposure-surface area to incident high-intensity solar-radiation, (b) by minimizing the effective "dead space" area of those parts (e.g., gaps between units) of the overall exposure area, which "dead space" areas do not contribute to cell response, and (c) by so proportioning the cell geometry as to assure a predetermined short maximum carrier-travel distance to the nearest p-n junction, thereby creating carriers with a high probability of being collected. Further detail is also provided in our paper, entitled "Performance of a new high-intensity silicon solar cell", Appl. Phys. Lett., Vol 34, No. 1, Jan. 1, 1979, pages 65-67. Disclosures of all the above-mentioned publications are hereby incorporated by reference.
In our U.S. Pat. No. 4,042,417, it is disclosed that cell response is optimal for incident radiation which is close to but offset from the p-n junction, but in said patent special lens elements were needed to concentrate the incident radiation within the desired region, i.e., close to but offset from the p-n junction, which region was 1 to 3 mils from the p-n junction for the silicon substrate there under consideration; the structures of said U.S. Pat. Nos. 4,128,732 and 4,129,458 make use of this optimal carrier-travel region with respect to the p-n junction, without requiring the special focusing lenses of said U.S. Pat. No. 4,042,417.
While all the single-wafer constructions of the above-identified publications rely upon micro-electronic manufacturing techniqes such as anisotropic etching, and selective doping to form p+ and n+ regions, there are certain complicating manipulative factors which necessarily become significant in the relatively high cost to manufacture the same. There is still therefore a need for such a construction which not only makes use of the indicated optimum range of carrier-travel distance to the p-n junction plane and otherwise maximizes the exposure-surface area having high probability of carrier collection at the nearest p-n junction, but which also represents simplification of manufacture, with attendant reduction in cost.